British Pat. No. 1,066,162 suggests subjecting a polyethylene terephthalate melt condensate [relative viscosity of at least 1.65 (measured in a 1% solution of the polyester in dichloroacetic acid at 25.degree.C), which corresponds to a polyester intrinsic viscosity of 0.54 dl/g (measured in a 1% solution of the polyester in phenol/tetrachloroethane in a weight ratio of 1:1 at 30.degree.C)] to polycondensation in the solid phase.
[Only intrinsic viscosity values are indicated in the following text, the correspondng values of relative viscosity may be read from FIG. 1, which shows the relation between the relative viscosity of a solution of 1% polyethylene terephthalate in dichloroacetic acid determined at 25.degree.C and the intrinsic viscosity of a solution of 1% polyethylene terephthalate in phenol/tetrachloroethane (1:1 per weight) at 30.degree.C.]
In the processes of the British patent, the reaction products which are volatile at polycondensation temperatures of 210.degree. to 235.degree.C (consisting mainly of ethylene glycol) are kept at a partial pressure of less than 2 mm mercury until the desired degree of polycondensation is reached. The adjustment of the partial pressure to a value of less than 2 mm of mercury is achieved by either
A. APPLICATION OF A VACUUM OR
B. MAINTAINING A GAS STREAM AT ATMOSPHERIC PRESSURE.
The streaming gaseous medium serves for the removal of the volatile reaction products and consists of a gas which is chemically inert to polyethylene terephthalate and which contains no free oxygen; carbon dioxide, sulfur hexafluoride, nitrogen or methane is suggested. When applying an inert gas stream, the inert gas is advantageously recirculated to the polycondensation vessel after separation of volatile reaction products therefrom.
The processes suggested in the British Patent Specification have a series of disadvantages. In order to achieve a high degree of polycondensation, as reflected by, e.g., an intrinsic viscosity of 1.10 dl/g, starting from intrinsic viscosity values of 0.67 dl/g and 0.85 dl/g, very long polycondensation times must be expected. Polycondensation times of more than 20 hours are the rule, especially in those cases where polycondensation is carried out under application of a vacuum. The desired intrinsic viscosity of 1.10 dl/g may be achieved faster by application of an inert gas stream under atmospheric pressure at simultaneous high streaming (of inert gas) rates. This advantage, however, is gained at the expense of increasingly elaborate devices and increased consumption of energy brought about by the maintenance of a high inert gas streaming rate and the elimination of volatile reaction products.